An investigation is made concerning the negatively charged environment, which the oxygen atoms of a glass provide for hosted metal ions, and the extent to which this changes on varying the glass composition. For Mn²⁺, d–d spectra show that the changes in the (average) oxygen electronic density of the glass are faithfully transmitted to the metal ion, and previous results for Pb²⁺ and Tl⁺ suggest that this probably operates generally for uni- and dipositive ions. This principle also appears to operate for Fe³⁺ in glasses where it is sixfold coordinated. However, in silicate glasses, where the coordination is fourfold, the sites occupied by Fe³⁺ are, surprisingly, of much lower electron density than the average for the oxygen atoms of the glass. The relevance of this study to the stabilisation of oxidation states for redox reactions, such as the Fe²⁺/Fe³⁺ equilibrium, is discussed in terms of the extent of electron donation in the Fe³⁺–O bonding and the charges residing on the Fe³⁺ species.